1. Field of the Invention
The present invention relates to an oscillator attachment structure used in e.g. a head end of CATV transmitter.
2. Description of the Related Art
A conventional oscillator attachment structure will be described with reference to FIGS. 4 to 6. An oscillator 31 has a box-shaped frame body 32 of metal plates having attachment legs 32a, a circuit board 35 having a wiring pattern 33 on its upper surface and a ground conductor 34 on its approximately entire lower surface, various electric components 36 soldered to the wiring pattern 33, connection pins 37 soldered to the wiring pattern 33 and projecting downward through the circuit board 35, and a metal cover 38 over the frame body 32.
Further, the connection pins 37 include plural power-source connection pins 37a provided in left and right positions, a tuning connection pin 37b, an output connection pin 37c and a ground connection pin 37d positioned between the power-source connection pins 37a. The connection pins 37 are not electrically connected to the ground conductor 34, being isolated from the ground conductor 34 by relief members 34a comprising rejection members provided in the ground conductor 34.
A printed circuit board 41 comprising a mother board that forms a head end of the transmitter holds a ground conductive member 42 on its upper surface, a wiring pattern 43 on its lower surface, and various electric components 44 soldered to the wiring pattern 43, and the.
Further, a buffer member 45 is formed with a rubber member 46 and adhesive 47 provided on the lower surface of the rubber member 46. The rubber member 46 is insulating and formed in a plate, i.e. planar, shape. The buffer member 45 is attached to the upper surface of the ground conductive member 42 of the printed circuit board 41 by the adhesive 47.
Further, a PLL circuit (not shown) is formed in the wiring pattern 43 opposite to the buffer member 45.
In the above-described oscillator 31, the attachment legs 32a of the frame body 32 and the power-source connection pins 37a are inserted into holes of the printed circuit board 41, and the tuning connection pin 37b, the output connection pin 37c and the ground connection pin 37d are inserted through holes of the buffer member 45 and the printed circuit board 41. The lower surface of the circuit board 35 is disposeddisposed on the rubber member 46. In this state, the connection pins 37 and the attachment legs 32a are soldered to the wiring pattern 43, thus the oscillator 31 is mounted on the printed circuit board 41.
The power-source connection pins 37a and the tuning connection pin 37b are not electrically connected to the ground conductive member 42 by the relief members 42a. 
In the oscillator attachment structure having the above construction, fluctuations of an oscillation waveform from the oscillator 31 due to shock and vibration from the printed circuit board 41 side, e.g. f skip, are prevented by the buffer member 45.
However, as the rubber member 46 of insulating material is used, a capacitance formed between the ground conductor 34 of the circuit board 35 and the ground conductive member 42 of the printed circuit board 41 is small. One result of this is that the circuit board 35 is not sufficiently grounded to the printed circuit board 41 in a so-called electrically floating state.
As a result, a reference oscillation signal from the PLL circuit formed on the printed circuit board 41 side influences the circuit board 35 side. The signal is mixed with the oscillation signal from the oscillator, which causes a beat signal as an interference signal.
The present invention has been made in consideration of the above situation, and provides an oscillator attachment structure which prevents an interference signal (more specifically) a beat signal and attains excellent performance.
According to one aspect of the present invention, the foregoing object is attained by providing an oscillator attachment structure comprising: a metal frame and a first circuit board having a first ground conductor on a lower surface thereof. The first circuit board is attached within the frame. A printed circuit board has a second ground conductor on an upper surface thereof. The oscillator is attached to the upper surface of the first circuit board. A conductive rubber plate, electrically connected to the first ground conductor, is provided on the lower surface of the first circuit board, and the conductive rubber plate is disposed on the second ground conductor. Further, according to another aspect of the present invention, the foregoing object is attained by providing an oscillator attachment structure comprising: a metal frame and a first circuit board having a first ground conductor on a lower surface thereof. The first circuit board is attached within the frame. A printed circuit board has a second ground conductor on an upper surface thereof. An oscillator is attached to an upper surface of the first circuit board. A thin insulating member disposed on the second ground conductor. A conductive rubber plate, electrically connected to the first ground conductor, is provided on the lower surface of the first circuit board, and the conductive rubber plate is disposed on the thin insulating member.
Further, according to another aspect of the present invention, the foregoing object is attained by providing the oscillator attachment structure, wherein the insulating member comprises a double-faced adhesive tape, and wherein the conductive rubber plate and the ground conductor are attached to each other by the double-faced adhesive tape.
Further, according to another aspect of the present invention, the foregoing object is attained by providing the oscillator attachment structure, wherein conductive grease is provided between the conductive rubber plate and the first ground conductor.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same name or similar parts throughout the figures thereof.